some scenes are more silvery/gray and some are more blue, but there is no way in the world that it could be considered white, the cap sleeves and whatever the puffs on her hips are, are white and there is a definite difference between them and the dress, no matter which of the colors it looks at the time.
I don't think it matters that her park dress is different from her movie dress. As a princess, she must have a huge wardrobe. A girl can't be expected to be seen in the same dress all the time, can she?